1. Field of the Invention
This invention pertains to a snap-on adaptor to standard triangular measuring rulers, such as those commonly used by architects, engineers, draftsmen, and persons in similar or related professions. Such adaptors can provide a variety of supplementary changeable scales and/or serve as ruler guards.
2. Description of the Prior Art
A search of the prior art relating to the present invention was performed in Class 33, subclasses 483 and 484 and Digest 16. Files of foreign patents corresponding to the listed class, subclasses, and digest also have been reviewed.
The search uncovered five U.S. patents (No. 3,310,878 of Mar. 28, 1967 to J. E. Giba; No. 2,504,251 of Apr. 18, 1950 to E. W. Coffey; No. 2,382,758 of Aug. 14, 1945 to W. D. Weeks and L. Kube; No. 564,342 of July 21, 1896 to L. K. Sherman; and No. 146,673 of Jan. 30, 1874 to S. Haslett) and Austrian Pat. No. 121,946 of Mar. 25, 1931 to H. Judtmann.
The Judtmann patent discloses a triangular non-transparent sheath of metallic or plastic material fitting over a triangular ruler and having one of its sides partly cut out or provided with a window to permit reading of only one selected scale, thereby preventing accidental use of an inappropriate scale. The angle of the sheath opposite said opening or window may be made smaller than the angles of the triangular rule, thereby causing the sides of the sheath to exert sufficient pressure on the ruler to prevent slippage.
The Haslett patent discloses an adjustable guard, made of metal, hard rubber, or other suitable material which can be fitted to a ruler by elastic pressure. The guard has two unequal sides, with a protruding part between these sides serving as a handle and preventing the ruler from being laid down the wrong way. The shorter of the two sides leaves the numerals of the selected scale exposed, whereas the longer side covers all the numerals, thereby again preventing accidental use of a wrong scale.
A guard similar to that of Haslett, but without the protruding part, and extending throughout the length of the graduated scales, is disclosed by Coffey. Its purpose is the same as that of Haslett or Judtmann.
Sherman's patent discloses an attachment to triangular rulers which serves to prick the measured points.
The device of Weeks et al. is for forming triangular rules out of a plurality of blades having a variety of graduated scales thereon.
Giba's patent discloses a slotted plate which can be attached by spring clips to a triangular rule for drafting purposes.
None of the cited patents disclose means for attaching a variety of adaptable scales to a traingular scale. Neither do any of these patents disclose a substantially flat upper part pressing against the upper edge of the ruler so as to provide a stable and comfortable rest cap for the palm of the hand.